The design and operating characteristics of monolithic crystal filters are basically described in U.S. Pat. No. 3,564,463. The aforementioned patent further describes the use of bridging capacitors connected between the input and output electrodes of the monolithic crystal filter element for increasing the passband selectivity of a monolithic crystal filter. For applications where it is desirable to have even further passband selectivity, two or more monolithic crystal filter elements with bridging capacitors may be cascaded. Such an arrangement of monolithic crystal filter elements is useful in the IF line up of radio receivers. In order to facilitate the interconnection of monolithic crystal filter elements, the resonating frequencies of the electrode pairs on the monolithic crystal filter elements are selected to be below the passband center frequency so that capacitive coupling may be utilized between successive monolithic crystal filter elements. However, the frequency response of these monolithic crystal filters is not symmetrically located with respect to the passband center frequency. This creates problems in applications, such as in highly selective radio receivers, where symmetrical selectivity is important.
In U.S. Pat. No. 3,716,808, the frequency response of a monolithic crystal filter with a bridging capacitor has been rendered symmetrical with respect to the passband center frequency by constructing the monolithic crystal filter element such that the resonating frequency of one electrode pair is higher than the resonating frequency of the other electrode pair. However, monolithic crystal filter elements having electrode pairs with different precisely controlled frequencies are more difficult to manufacture than, and consequently more expensive than, monolithic crystal filter elements having electrode pairs with identical frequencies.